The present application relates to phosphor tape articles and specifically to phosphor tape articles that include a pressure sensitive adhesive layer.
Many white light sources that utilize LEDs in their construction can have two basic configurations. In one, referred to herein as direct emissive LEDs, white light is generated by direct emission of different colored LEDs. Examples include a combination of a red LED, a green LED, and a blue LED, and a combination of a blue LED and a yellow LED. In another basic configuration, referred to herein as LED-excited phosphor LEDs, a single LED generates a beam in a narrow range of wavelengths, which beam impinges upon and excites a phosphor material to produce visible light. The phosphor can include a mixture or combination of distinct phosphor materials, and the light emitted by the phosphor can include a plurality of narrow emission lines distributed over the visible wavelength range such that the emitted light appears substantially white to the unaided human eye.
One method for producing white (broad-spectrum) light from LEDs is to combine a blue or ultraviolet (UV) emitting LED with a suitable phosphor or blend of phosphors. In practice, phosphor powder is either coated directly onto the LED die or dispersed in the polymer encapsulant surrounding the LED die. One example of a white light LED is formed by combining a blue light emitting InGaN LED encapsulated with a yellow emitting phosphor such as cerium-doped yttrium aluminum garnet (YAG:Ce). White light is produced by this combination when the right amount of phosphor is deposited over the blue die to absorb most, but not all, of the blue light such that unabsorbed blue light and yellow light combine to create light that appears white.
Producing packaged white LEDs having a consistent white color requires careful control of the quality of phosphor deposited over the LED. Too little phosphor causes the LED emission to appear blue white while too much phosphor causes the emission to appear yellow. Accurately controlling the amount of phosphor deposited is difficult as package size (volume) decreases. The encapsulant dispensing system must have a dispensing precision on the order or tens of nanoliters and high phosphor loadings are difficult to dispense due to the non-Newtonian flow behavior of the phosphor loaded resins and clogging of the dispenser from phosphor agglomerates.
Incorporating the phosphor in the encapsulant also presents problems with manufacturing yield and quality control. Due to variation in semiconductor manufacturing and variation in the quality of encapsulant deposited in the LED package, each white LED must be tested and measured after encapsulation to determine its individual color and brightness. Based on this testing, the white light LEDs are sorted into bins. White LEDs that are too far out of tolerance cannot be reworked and are scrapped. Bubbles trapped in the encapsulant and inhomogeneous distribution of phosphor are other problems that are inherent in current white LED production and lead to additional yield loss.